Of the known methods for synthesis of sodium aluminum hydride (NaAlH.sub.4), the so-called "direct synthesis" process appears to offer the most promise. It involves reacting Na (or NaH), Al and H.sub.2 under suitably elevated temperatures and pressures in an appropriate liquid reaction medium, such as an ether, amine or hydrocarbon. See in this connection:
--E. C. Ashby, Chem. Ind. (London), 208 (1962); PA1 --E. C. Ashby et al., Inorg. Chem. 2, 499 (1963); PA1 --E. C. Ashby, French Pat. No. 1,235,680 (May, 1960), and U.S. Pat. Nos. 4,045,545 and 4,081,524; PA1 --J. Murib, U.S. Pat. No. 3,556,740; PA1 --H. Clasen, Angew. Chem. 73, 322 (1961).
It has been suggested heretofore to conduct the direct synthesis reaction in such a way that the heel from one run is used to initiate the reaction in an ensuing run. See E. C. Ashby et al. (loc. cit.) at page 501. Nevertheless, the data presented in their Table IV indicate that a reaction conducted with aluminum recycled from a prior reaction was somewhat slower than a reaction conducted under the same conditions using aluminum powder activated by means of triethyl aluminum. And the recycled aluminum required a seven hour reaction period to achieve a 93 percent conversion of Na to NaAlH.sub.4 even though the process was conducted at 140.degree. C. and 2000 psi in diglyme.
In commercial operations, time is of the essence. The greater the reactor throughput, the lower the cost. A desirable contribution to the art, therefore, would be a way of increasing reactor throughput in the direct synthesis reaction.
Effective utilization of raw materials is also of paramount importance in commercial operations, especially where the raw materials are relatively expensive and the disposal of their residues is potentially hazardous and troublesome. Thus another desirable contribution to the art would be the provision of a direct synthesis process which not only exhibits increased reactor throughput but which achieves effective utilization of aluminum and sodium and avoids problems associated with their disposal.
It is believed that this invention fulfills the foregoing objectives in an efficient and economical manner.